This invention generally relates to power supplies for electroluminescent (EL) devices and more particularly to a power inverter for low voltage direct current (DC) sources and which includes the EL device as part of an oscillator resonant circuit such that changes in EL capacitive reactance directly change the inverter oscillation frequency. The change in oscillation frequency compensates the change in EL capacitance and maintains a high light output.
Electroluminescent devices are generally constructed as planar light emitting sources with luminescent phosphors suspended in a dielectric material and energized by a high electric field. The electric field typically is applied by parallel conducting plates disposed on each surface of the planar dielectric material and coupled to opposite polarities of a voltage source. One of the plates is optically transparent or translucent so that light may escape from one surface of the planar device, thereby producing an essentially flat light source.
Such a flat light source has found application as a night light for home use and other low illumination applications. This light source has not found wider acceptance because of its inefficiency in converting electrical energy into light energy, its large capacitive reactance, and its variation in light output as the EL device aged.
Generally, the electrical energy applied to an EL device is alternating current (AC) electrical energy. Supplying the EL device from line current fixes the frequency of the AC energy. To improve the efficiency, others have used a reactance cancelling inductor so that a majority of the applied power would be delivered to the resistive component of the EL device. Additional efficiency has been achieved by increasing the electric field across the EL device by supplying power to the EL device at a frequency close to the resonance point of a moderate Q resonant circuit formed by the EL device capacitive reactance and the inductor. However, as the device capacitance changes with age, the resonance frequency of the resonant circuit moves away from the supplied power frequency and the light output diminishes considerably.
Powering the EL device from a direct current (DC) power source requires that the DC power be converted to AC power by means of a power inverter. Power inverters are well known and commercially available for many purposes. Typically, a power inverter switches or chops the DC power at a rate convenient for operation of AC devices--for example 60 Hz--and provides a switched AC output. To improve the efficiency of EL devices, under these circumstances the frequency of the power source has been made variable and dependent upon the capacitance of the EL device. In one previous embodiment, the switching rate of a DC power inverter is dependent upon the charge time of the EL device capacitance. In another, the current drawn by the EL device is detected and a control signal generated to vary the frequency of a controllable oscillator in order to hold the current constant.
None of these methods have yielded an efficiency or a constancy of light output which would enable the use of an EL device in a battery powered portable product. Such a product may use an EL device for the purpose of illuminating a telephone dial or other surface such as that envisaged in U.S. Pat. No. 4,206,501 assigned to the assignee of the present invention. The present invention produces the efficiency and constancy necessary for such illumination purposes.